1. The present invention relates to the field of injection molding, most particularly to the precision formation of wax patterns having ceramic cores for use in the "lost wax" method of metal casting.
2. The present invention, and much of the prior art, are uniquely related to the process of casting superalloys into the form of gas turbine airfoils. Airfoils, such as blades and vanes, are components characterized by very thin walls. To form such components using the low wax method, a wax pattern containing a thin and highly precise ceramic core must be made, typically by injection molding. The ceramic core of course determines not only the internal dimensions, but the thickness of the walls of the pattern and resultant cast part. Consequently not only must the core be made to an accurate dimension, but it must maintain its location during both the formation of the wax pattern and during the casting process. A typical method in the art is to grip the core at selected end points. However, in many turbine blades this is often only possible at one end; this coupled with the typical thin core cross section results in frequent core shifting. Schemes have been used to overcome this propensity, such as disclosed by Bishop et al in U.S. Pat. No. 3,662,816 where metal pins are inserted into the wax pattern after the pattern is formed around the core. These protruding pins are then incorporated into the shell mold formed around the pattern and thereby hold the core in place until the metal of the casting is introduced into the mold and sweeps them away.
Ceramic cored airfoils have been made for a number of years. But in the recent decade there has been demand for increasingly thin central passages (cores) and walls and highly complex configurations on the core surface and resultant airfoil interior. Strong, stable and precise cores, removable without damage to the cast article, are sought but the inherent technical conflicts limit the article quality. Deviations attributable to the core failures have been heightened also by prolongation of the casting time and temperature in newer directional solidification processes.
A significant technological strategy to overcome these problems involves the casting of an article around a thicker core, often called a strongback, followed by the separation of the article into two halves and the subsequent rejoining of the halves to form a gas turbine airfoil. The methods and rationales of this new approach are detailed in the patents mentioned below and the references therein.
But even though the use of a thicker core overcomes some of the problems, it is still necessary to precisely locate the core within the metal injection molding die. Several U.S. patents of the present assignee have disclosed previous approaches which have been taken to precisely locate the core. Phipps et al U.S. Pat. No. 3,965,963 and Hayes et al U.S. Pat. No. 3,981,344 do not address the means of holding the core during wax injection but do show how the core is held during casting, namely by means of the longitudinal flanges which also serve to define the bonding planes of the blade halves. Herold in U.S. Pat. No. 4,068,702 discloses a core held by higher temperature wax pins which are placed within the die on either side of the core, in combination with the edge flanges. The pins mate with raised and depressed portions of the core which also serve to provide corresponding points on the opposing blade halves to facilitate their subsequent location during bonding. Kelso and Obrochta (one of the present inventors) in U.S. Pat. No. 4,078,598 disclose another approach wherein the core is located in the injection molding die by pins which are colinear with and in close proximity to the bonding locators. Cooperating with these locators are fixed or retractable pins within the die. In both the foregoing Kelso et al and Herold disclosures, the pins are located generally along the longitudinal centerline of the core, while the core is also held by flanges at its periphery.
Now with further development of the total process of manufacturing turbine blades, new improvements on the foregoing approach were needed and are revealed herein. Control of the wall thickness of the casting is desired within tolerance of better than .+-.0.012 mm, for thicknesses of from 0.5 to 1.3 mm. The ceramic cores are made by an elaborate and costly process of consolidation and firing. While these processes are quite advanced they are nonetheless prone to producing cores which have unavoidable deviation both in thickness and contour, of the order of the sought tolerance. Consequently it has not in practice been found feasible to hold the cores between precise fixed locating points within the die without encountering an unacceptable degree of core breakage if the locating point spacing is set too tight, or shifting of cores on the other hand if the tolerance is set loose enough to accommodate the maximum core deviation. It should also be appreciated that the forces of injection molding of the wax are appreciable and thus the location of the core within the die must be both positive and adequate to resist change during injection molding. (The invention of Myllymaki "Injection Molding Thermoplastic Patterns Having Ceramic Cores" disclosed in application Ser. No. 136,599, filed on even date hereof, involves improved means for injecting wax into die containing cores, and has a relation to the present invention.)
Thus, there has been a need for improved location of cores within metal dies for injection molding. Further, since the present state of the art of ceramic core manufacture is that there will be inevitable deviations, there is a need for a method of getting the best part yield therefrom. That is, if there are to be deviations in a wax pattern and a gas turbine type article made therefrom, what is the best way of accommodating these deviations?